Mg + 2HCl -> H2 + Mg2+ (aq) + 2Cl- (aq) how many h2 moles?

Question

I need help with my lab due 2morrow! How many moles of H2 are in this reaction? I have been trying to figure this out for days now!

Answer 1

Mg + 2HCl --> H2 + Mg2+ + 2Cl-

How many H2 moles?

Are we missing something?  Based on the balanced chemical equation, one mole of hydrogen gas, H2, is produced for every mole of magnesium that reacts.

For instance, if 10.0 g Mg reacts with excess HCl, then ...

10.0 g Mg x (1 mol Mg / 24.3 g Mg) x (1 mol H2 / 1 mol Mg) = 0.412 mol H2

============= Follow up ============

I suspected that there was more to the problem, and just pulled 10 g of Mg of the top of my little bald head.  But it seems that it is exactly the situation.  If all of the Mg reacts, it is because there is an excess of HCl.  But it appears that the H2 is liberated to the atmosphere.  How are you supposed to measure the amount of H2 produced?  Compute it?

Again, look at the example I gave you:

10.0 g Mg x (1 mol Mg / 24.3 g Mg) x (1 mol H2 / 1 mol Mg) = 0.412 mol H2

If you use 24.305 (instead of 24.3, as I did) you get 0.411438 mol H2.  But that's too much precision and too many significant digits for 10 grams, but that's another issue.  Why did you get a value that is 1000 times too small?

Is there a missing step 5?

Take a look at this tutorial.   I think it will help.

http://www.chemcollective.org/stoich/rea...

All of the material on stoichiometry may be found here:

https://oli.web.cmu.edu/jcourse/lms/stud...

Answer 2

i think this question is missing a little. You need Mg or HCl information